Implement handle on non-linux platforms
Implement handle on non-linux platforms

When developing for Linux on a non-Linux platform, it's useful to have
mock method implementations so the code can easily build locally.
This patch adds such mock implementations for the handle struct, as is
already done in netlink_unspecified.go and route_unspecified.go.

vishvananda/netlink,porjo/netlink,axsh/netlink,paravmellanox/netlink,mcastelino/netlink